Physics

Electromagnetic Waves and Spectrum

659 Questions

Electromagnetic waves and spectrum questions test a candidate's understanding of radiation frequencies, wavelengths, and the properties of different rays like infrared, ultraviolet, and visible light. Concepts also cover practical applications in astronomy and the fundamental speed of light calculations. This physics topic appears regularly in general science sections of major competitive examinations.

UV rays propertiesElectromagnetic radiation frequencyWavelength identificationSpeed of light calculationsBlack body radiation

Electromagnetic Waves and Spectrum Questions

Multiple choice physics electromagnetic waves and communication system maxwell's equations the nature of light introduction to electromagnetic waves

Fill the blank space with the best suitable option. All electromagnetic waves have the same _________ while travelling in a vacuum.

  1. amplitude

  2. frequency

  3. wavelength

  4. Intensity

  5. speed
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

All electromagnetic waves (EM waves) travel with the speed of light in vacuum but these have different frequency, amplitude, wavelength and intensity.

Thus option E is correct.

Multiple choice physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

Which of the following is responsible for passing the energy from one to the another to transmit the light waves.

  1. atom

  2. neutron

  3. fiber

  4. object

  5. wavelength
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Answer A is correct 

Atom is responsible for transmission of light.
By collision of atoms they passes energy from one to another and hence light is transmitted. 

Multiple choice physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

Identify which of the following best describe the difference between electromagnetic (EM) waves and other types of waves?

  1. EM waves can travel without a medium.

  2. EM waves are higher in frequency than all other waves

  3. EM waves have shorter wave lengths than any other type of wave.

  4. EM waves transport matter and energy.

  5. EM waves require a medium to travel.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

EM waves do not require a material medium to travel, these can propagate even through vacuum while all other waves require a certain medium to travel.

Multiple choice physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

In vacuum, electromagnetic waves travel at the speed of

  1. $3 \times {10}^{8} {m}/{s}$
  2. $3 \times {10}^{6} {m}/{s}$
  3. $3 \times {10}^{-8} {m}/{s}$
  4. $3 \times {10}^{18} {m}/{s}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In vacuum, electromagnetic waves travel at speed of $3\times { 10 }^{ 8 }$ m/s.

C = speed of EM waves
C = $\frac { 1 }{ \sqrt { { \mu  } _{ 0 }{ \varepsilon  } _{ 0 } }  } $
                ${ \mu  } _{ 0 }$ = permaebility of free space
                ${ \varepsilon  } _{ 0 }$ = permittivity of free space
putting the value of ${ \mu  } _{ 0 }$ and ${ \varepsilon  } _{ 0 }$ we get value of C approximately $3\times { 10 }^{ 8 }$ m/s.
                ${ \mu  } _{ 0 }$ = $1.257\times { 10 }^{ -6 }$ Henry/meter
                ${ \varepsilon  } _{ 0 }$ = $8.85\times { 10 }^{ -12 }$ Farad/meter

Multiple choice physics observing space: telescopes maxwell's equations the nature of light introduction to electromagnetic waves

The electric field of a plane electromagnetic wave is given by
$\vec{E} = E _0 \dfrac{\hat{i} + \hat{j}}{\sqrt{2}} \cos (kz + \omega t)$
At $t = 0$, a positively charged particle is at the point $(x, y , z) = \left(0, 0 , \dfrac{\pi}{k} \right)$. If its instantaneous velocity at $(t = 0)$ is $v _0 \hat{k}$, the force acting on it due to the wave is :

  1. parallel to $\hat{k}$
  2. parallel to $\dfrac{\hat{i} + \hat{j}}{\sqrt{2}}$
  3. antiparallel to $\dfrac{\hat{i} + \hat{j}}{\sqrt{2}}$

  4. zero
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\vec{E} = E _0 \left(\dfrac{i + j}{\sqrt{2}}\right) \cos (kz + wt)$


$\therefore$ unit vector along electric field, $\vec{E} = \left(\dfrac{\hat{i} + \hat{j}}{\sqrt{2}} \right)$


Direction of electromagnetic wave is in (-z) direction 

$\therefore \hat{C} = -\hat{k}$  wave direction.

Let $\hat{B} $ is the unit vector along the direction of magnetic field.

$\hat{B} = \hat{C} \times \hat{E} = -\hat{k} \times \left(\dfrac{\hat{i} + \hat{j}}{\sqrt{2}} \right) = - \left(\dfrac{\hat{k} \times i + \hat{k} \times j}{\sqrt{2}} \right)$

$\hat{B} = -\left(\dfrac{\hat{j} + (-i)}{\sqrt{2}} \right)  = \left(\dfrac{\hat{i} - \hat{j}}{\sqrt{2}} \right)$

$\vec{F _e} =$ electric force on the charge particle 

$\vec{F _e} = $ unit vector of electric force $= \dfrac{q \hat{E}}{|q\hat{E}|} = \hat{E}$

$\vec{F} _e = \dfrac{\hat{i} + \hat{j}}{\sqrt{2}}$

$\vec{F} _b = $ magnetic force $= q \vec{V} \times \vec{B} = q \left(V _0 \hat{k} \times \dfrac{i - j}{\sqrt{2}}\right)$

$\vec{F} _b = q V _0 \left[\dfrac{\hat{k} \times \hat{i} - \hat{k} \times \hat{j}}{\sqrt{2}} \right] = q V _0 \left[\dfrac{\hat{j} - (-\hat{i})}{\sqrt{2}}\right]$

$\vec{F} _b = q V _0 \left(\dfrac{\hat{i} + \hat{j}}{\sqrt{2}} \right)$

$\vec{F} _{Net} = \hat{F} _e + \hat{F} _b = \dfrac{\hat{i} + \hat{j}}{\sqrt{2}} + \dfrac{\hat{i} + \hat{j}}{\sqrt{2}} = \dfrac{2}{\sqrt{2}} (\hat{i} + \hat{j})$

$\therefore \hat{F} _{Net} = $ unit vector $= \dfrac{\hat{i} + \hat{j}}{\sqrt{2}}$

Option (B) is correct.

Multiple choice dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

If $C$ is the value of the capacitance of a capacitor filled with a given dielectric and $a$ is the capacitance of an identical capacitor in a vacuum, the dielectric constant, symbolized by the Greek letter kappa, , is simply expressed as

  1. $\dfrac{C}{a}$
  2. $\dfrac{a}{C}$
  3. $C\times a$

  4. None

Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The capacitance of a parallel plate capacitor filled with the given material of dielectric constant $\kappa$ is given by:

              $C=\dfrac{\kappa\varepsilon _{0}A}{d}$ .....................eq1
And the capacitance of the same capacitor without the given material is given by:
              $a=\dfrac{\varepsilon _{0}A}{d}$  .....................eq2
Dividing eq1 by eq2:
              $C/a=\kappa$

Multiple choice dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

The insertion of a dielectric between the plates of a parallel-plate capacitor always 

  1. increases its capacitance

  2. decreases its capacitance

  3. remains same its capacitance

  4. none of the above

Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The capacitance of a parallel plate capacitor completely filled with a given material of dielectric constant $K$ is given by:

              $C=\dfrac{K\varepsilon _{0}A}{d}$ .....................eq1
And the capacitance of the same capacitor without the given material is given by:
              $C'=\dfrac{\varepsilon _{0}A}{d}$  .....................eq2
Dividing eq1 by eq2:
              $C/C'=K$
As $K>1$ always, hence $C>C'$ i.e. the insertion of a dielectric between the plates of a parallel -plate capacitor always increases the capacitance.

Multiple choice physics effects of light scattering of light and its applications some natural phenomena of light natural phenomena explained by scattering

In context of scattering
Fine particles : ____ wavelength : : large particles : _____ wavelength

  1. Long, short

  2. Short, long

  3. Visible, infrared

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Fine particles scatter light of shorter wavelength at the blue end of the spectrum and larger particles scatter light of longer wavelength at the red end of the spectrum.

Multiple choice physics wave optics huygens wave theory and wavefront wave propagation (huygens' construction) theories on light wave behaviour

If light waves emitted by an ordinary source, for what period of time, the phase remains constant :-

  1. $10 sec$
  2. $1 sec$
  3. $10^{-3}$ sec
  4. $10^{-8}$ sec
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In ordinary light sources, light is emitted due to independent atomic transitions lasting roughly 10^-8 seconds, during which the phase remains relatively constant before a sudden random phase change occurs.